Historically hydrogen dioxide (30%) has been used to etch titanium tungsten (TiW). The etchant has been employed because of its selectivity to other materials and its less corrosive nature than alternative etchants. The etch rate is slow, so the fluid is typically heated to 40° C. to increase the etch rate. Although the process results can be excellent, the heated hydrogen dioxide presents a number of process and safety hurdles to overcome.
Hydrogen dioxide degrades naturally and this degradation is accelerated with an increase in temperature. The degradation is the molecule splitting into water and oxygen gas. When this occurs inside vessels or other plumbing, vapor pockets form within the liquid. Liquid dispenses will then be partially liquid and partially vapor and this can greatly affect process results. It takes some time to heat and stabilize the etchant loop so during standby condition a process tool needs to maintain the fluid in circulation and at temperature. This rapidly degrades the chemistry in the standby mode, even with no production occurring. The slow etch rate (even if heated) means the processes are fairly long in duration. Accordingly the chemistry needs to be recycled to make the process economical. The material to be etched normally coincides with a range of materials. Some of these could be transitional metals or other material that will greatly increase the degradation rate of hydrogen dioxide. This can lead to safety issues where the liquid will rapidly decompose and over pressurize plumbing components to an unsafe condition.